The best thing in the world to do is right here. Take a 2 Liter of Sprite or 7-up. Clear soda so they can't detect it. Put it in the 5 gallon gas can, fill the rest of the way with fuel. When they steal this and put it in their vehicle, that will be the end of the fuel system and the engine, The sugar will gum up/kill the pump, regulator and injectors. Then valve guides become gummed up and cause the valves to hang open and the engine will randomly misfire and run horrible. New engine(Or atleast heads) and fuel system and a serious fuel line and tank flush will be in order. This would total any vehicle over a few years old just in the cost to fix it. I was a Ford technician for many years before I got into this industry. Back in 1999 or so while I was still working for the Ford dealer I was called to the service lane one morning for an irrate customer that had a new truck that was misfiring and he was raising hell since the truck only had 900 miles on it and he wanted it fixed right now. So i took it in the back and retrieved the codes. Had misfire codes. I ran a power balance and watched each cyclinder misfire and then it would run fine on that cylinder. They were dropping out two to three at a time and then they would stop misfiring and a different two cylinders would do the same thing. So, im standing there and checking injector connectings and Pcm connections and grounds. Totally scratching my head? Like WTF is going on here? An hour or so later after checking a few other things I took a fuel sample.....and there it was...
I pulled the injector rail and it was just a mess. Pulled the gas cap and saw some residue in the filler neck. Brought the customer back and showed him and he quickly stopped being an azz. Called his insurance and like 9300.00 he was all fixed up. About 10 days later some one on another team in the dealership came back to my shop and asked me what my truck was doing when I diagnosed it, since he had one that was acting the exact same way. I told him to go pull the fuel rail off and check the injector nozzles. Same thing...It was the rental F-150 the my guy had rented from Budget Rent A Car while I was fixing his truck. He ended up having to pay out of pocket for the rental repairs since his insurance denied the claim and dropped him. So, if you want to get someone that is stealing your fuel.....there you go. Putting water in it will only kill the pump and injectors possibly. Soda and gas equals one unhappy and unmobile thief.....
Oh, and I did put a locking gas cap on it before it left my sight....

Same thing w/ me dog going nuts, I got hit this year, they tore the fuel tank off my z spray and stole a 6 gal jug, left the redmak 8000 series blower. Everything goes inside now. Turns out they were hitting my trucks my wifes truck and equip for a while.

A friend told me I should string a line from a tree to my garage with fish hooks hanging around eye level, if I knew I couldnt get sued I would have done it.